The present invention relates generally to liquid crystal polymer (LCP) film laminates, and more specifically to flex circuits comprising LCP substrate films laminated to a cover-layer of LCP film to encapsulate conductive circuit traces and protect them from environmental contamination.
Thermal inkjet printers are fast, quiet devices that produce high quality printing by ejecting ink onto an image-recording medium. An image-recording medium, such as a sheet of paper, is struck only by ink as the printhead moves over the paper surface during image formation.
Thermal inkjet print cartridges operate by ejecting vaporized ink through one or more orifices to produce an ink dot on the recording medium. Ink ejection occurs quickly following rapid heating of a small volume of ink. Orifices may be arranged in arrays on a nozzle plate. As the printhead moves across the recording medium, a sequenced ejection of ink results in a printed portion of the desired image. Typically, after each pass of the printhead, the recording medium moves to allow the next printhead pass to deposit ink onto the adjacent area, until a complete image appears.
A typical inkjet print cartridge includes a printhead, an ink reservoir for containing liquid ink, which is delivered to the printhead, and a flex circuit for transmitting signals to the printhead. The printhead may be any of a variety of conventional structures, including those with nickel nozzle plates, and the like, or may be formed using Tape Automated Bonding (TAB).
Damage to a printhead cartridge may occur in several ways. A common problem involves attack by the ink jet ink composition on the surfaces of the ink jet printhead chamber and nozzles. Attack on the surfaces, by erosion, occurs before and during ink droplet ejection. The process of droplet ejection occurs at temperatures of about 300xc2x0 C. Such a high temperature may accelerate wear occurring in the printhead. Protective coatings may extend printhead lifetime by retarding the rate of erosion.
It is known to use of organic polymers to protect flex circuitry from environmental agents such as dust and humidity. Although unsuitable for preventing printhead erosion, organic polymers are useful for protecting structures such as printed circuit interconnects in the proximity of an ink jet printhead from environmental agents such as dust and humidity and especially from ink jet ink spray. Ink spray, present during the operation of an inkjet printer, can deposit as ink droplets on interconnects comprised of electronic signal carrying conductive traces, usually made of copper. Droplets of ink contain ionic components. Spacing between individual circuit traces is such that an ionically conducting ink droplet could span the gap between adjacent traces causing an electrical short. In the presence of an electrical short, the inkjet printer may malfunction. Obviously an electrical short is much more likely to develop if circuit traces are exposed without a protective electrically insulating coating. Even with such a protective coating, the deposition of ink droplets in the region of conductive circuit traces may eventually cause electrical shorting between traces. This is possible because ink jet inks contain solvents as well as ionic components. Solvents in the ink will gradually dissolve certain insulative coatings to expose the underlying circuit traces. Solvent attack may be readily facilitated by surfactants also present in ink jet inks. Information on ink jet ink compositions is available by reference to e.g. JP 3097771, U.S. Pat. No. 4,853,037, 4,791,165, 4,786,327, EP 259001, U.S. Pat No. 4,694,302, 5,286,286, 5,169,438, 5,223,026, 5,429,860, 5,439,517, 5,421,871, 5,370,730, 5,165,968, 5,000,786 and 4,990,186.
Problems associated with deterioration of ink jet chambers and nozzles affect the useful service lifetime of an ink jet cartridge. The problem of electrical shorting between circuit traces, of interconnecting flex circuits, poses a more significant problem of premature failure of a printhead when the signal control circuit becomes compromised. Failure of this type primarily involves conductive traces formed on films of polyimide which is the most common substrate polymer used for flex circuits. Desirable properties of polyimide polymers for this application include high dielectric constant and thermal stability. Conductive traces on polyimide typically require a protective coating layer to prevent corrosion by a variety of contaminants including inkjet ink spray. For optimal protection, covercoat materials resist chemical attack, avoid contamination by outgassing and adhere well to conductive circuit traces and the polyimide circuit supporting substrate. A major drawback to firther progress using polyimide film relates to the way in which polyimide absorbs moisture to levels that interfere with high frequency device performance. Higher frequency operation requires the identification or development of electronic packaging materials with less susceptibility to moisture absorption.
Liquid crystal polymer (LCP) films find increasing use as substrates for flexible circuits having improved high frequency performance. Generally they have lower dielectric loss, and absorb less moisture than polyimide films. These beneficial properties of liquid crystal polymers were known previously but difficulties with processing prevented application of liquid crystal polymers to complex electronic assemblies.
The development of multi-axial, e.g. biaxial, film processing techniques expanded the use of liquid crystal polymer film for flexible circuit applications. U.S. Pat. No. 4,975,312 describes a printed wiring board substrate prepared from a multi-axially oriented thermotropic liquid crystalline polymer film having a tailored coefficient of thermal expansion in the X-Y direction and a thickness of not more than about 100 xcexc. Materials of this type offer several potential advantages over the use of polyimide films for flex circuit substrates. Such potential advantages led to the use of readily available processing techniques for producing single layer or multilayer circuit structures supported by one or more layers of a liquid crystal film substrate. A multilayer flexible circuit is a combination of three or more layers of single or double-sided flexible circuits laminated together and processed with drill and plating to form plated through-holes. This creates conductive paths between the various layers without having to use multiple soldering operations.
U.S. Pat. No. 4,737,398 describes a laminated film comprising a metallic foil and a polymer layer laminated to the metallic foil. Metal foils in the range of 0.005-0.076 mm may be laminated to an anisotropic melt phase forming polymer to provide a polymer laminate 0.01-0.254 mm thick. The resulting laminates provide substrates for a variety of applications including electric and electronic wiring.
It is known that liquid crystal polymer films may be laminated together, or laminated to other materials. According to U.S. Pat. No. 5,248,530, to obtain good adhesion this process involves heating the LCP until it melts and flows to produce a bond between the laminated layers. Unfortunately, when an LCP is heated above its softening or melting point its molecular orientation and overall shape tend to change as the polymer flows. This problem may be overcome, as described in U.S. Pat. No. 5,248,530, by use of a material comprising a coextruded liquid crystal polymer (LCP) film or sheet wherein a higher melting LCP layer is enclosed in or sandwiched between layers or lower melting LCP. The LCP components are coextruded from the same die. This film or sheet may then be laminated on to other materials at a temperature between the melting points of the outer lower-melting LCP and the inner higher melting LCP. Under the conditions indicated above, the inner LCP maintains it shape, orientation and mechanical characteristics while the outer molten LCP component flows and bonds with the other material. Another laminate structure, described in European Patent EP 07646515, provides a composite material having at least three distinct regions formed by an outermost region of porous polytetrafluoroethylene(PTFE), an inner region of a liquid crystal polymer and an intermediate region produced by penetration of the LCP into the porous PTFE. Lamination of PTFE to LCP uses a heated platen press operating at temperatures in the range of 10xc2x0 C. to 50xc2x0 C. above the melting point of the thermotropic LCP. The composite structure forms with the LCP in a molten condition.
Liquid crystal polymer materials have received little attention as protective and encapsulating materials for electrical and electronic circuits and devices. German Patent DE 198 38 266, describes an apparatus and process for encapsulation of electrical circuits by means of an injection casting process. Japanese Patent JP 2000068263 uses an insulating LCP to cover the surface of an integrated circuit device such as a bare chip in a chip scale package. There is a clear demarcation boundary where the LCP layer contacts the surface of the chip. Since the packaging of most electronic devices falls either by delamination or circuit metal corrosion, any separation, along the boundary, between the chip surface and the LCP layer provides a point of access for fluids and other agents that may compromise the integrity of a circuit device.
In view of the deficiencies associated with known electronic packaging materials, the present invention has been developed to alleviate these drawbacks and provide further benefits to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.
The present invention in its several disclosed embodiments provides a composite structure useful for packaging flex circuits for use with electronic devices. Preferably flex circuits, according to the present invention, comprise a LCP film substrate and a printed circuit laminated to a protective cover film of a liquid crystal polymer. Laminated structures disclosed herein provide improved performance compared to conventionally packaged electronic circuit devices including polyimide substrates. The present invention also facilitates electrical interconnection while preserving hermeticity to reduce the exposure of the conductive circuit traces to environmental contaminants that could compromise circuit integrity.
LCP film packages provide improved performance when compared with polyimide flex circuits. For example, the low moisture absorption, low gas transmission rate, excellent dimensional stability, and low coefficient of thermal expansion of LCP minimizes the potential for circuit contamination and dimensional stress during circuit assembly and operation. Low dielectric constant and low loss tangent characteristics allow LCP to operate effectively at frequencies, such as microwave frequencies, that extend beyond the useable range for polyimide flex circuits. Packaged flex circuits using LCP film laminates become substantially hermetically sealed due to adhesion and bond formation between polymer and metal during the timed application of heat and pressure. A packaging method according to the present invention produces a bonded composite that is highly resistant to attack such as by ingress of contaminants. Hermetic seal formation during the time of application of heat and pressure promotes infusion of LCP films, removing any evidence of a demarcation boundary between the films, even at temperatures at or below the melting point of the LCP. Infusion occurs in the absence of dimensional change of the circuit structure.
More particularly the present invention provides a composite comprising at least a portion of an electrical conductor having a first surface opposite a second surface. A first homogeneous polymeric film contacts the first surface of at least a portion of the electrical conductor. A second homogeneous polymeric film contacts at least a portion of the first film and the second surface of the at least a portion of the electrical conductor. The first and second films infuse to form a continuous phase surrounding the at least a portion of an electrical conductor.
Another embodiment according to the present invention provides a flexible circuit including hermetic sealing using a composite comprising at least a portion of an electrical conductor having a first surface opposite a second surface. A first homogeneous polymeric film is in contact with the first surface of the portion of the electrical conductor. A second homogeneous polymeric film contacts at least a portion of the first film and the second surface of the portion of the electrical conductor and the first and second films infuse to form a continuous phase surrounding the portion of the electrical conductor.
The present invention also includes a process for producing a flexible circuit including hermetic sealing comprising the steps of providing at least one electrically conductive trace having a first surface opposite a second surface. At least a portion of the first surface is attached to a first homogeneous polymeric film. A second homogeneous polymeric film is contacted with at least a portion of the first film and at least a portion of the second surface. Thereafter, application of pressure to the first and second films, for a time at a selected temperature, promotes polymer infusion to form a continuous phase surrounding at least a portion of an electrically conductive trace.
Definitions
The following definitions provide clarification of terms used herein for describing the present invention:
xe2x80x9cCircuit Protective Compositexe2x80x9d refers to the composite structure formed according to the present invention by laminating a homogeneous polymeric film as a coverlayer over the circuit traces of a printed circuit supported on a similar homogeneous polymeric film substrate. Lamination to form the composite occurs during timed application of heat and pressure to the films to produce a continuous phase around the circuit traces and an infusion layer having no clear demarcation boundary between the film layers.
The term xe2x80x9chomogeneous polymeric filmxe2x80x9d refers to a film layer suitable for use to form a composite according to the present invention. The properties of the film are the same throughout the layer and the film has a single melting temperature. Homogeneous polymeric films may contains fillers and other additives as required for preferred embodiments of the present invention.
Use of the term xe2x80x9chermetically sealedxe2x80x9d or xe2x80x9csubstantially hermetically sealedxe2x80x9d indicates the existence of a material barrier to prevent access to protected structures, such as electrical conductors, by ingress of agents that may corrode or otherwise damage or compromise the function of a protected structure. An hermetically sealed package is not viewed as being completely impermeable due to the fact that the preferred liquid crystal polymer films used according to the present invention are subject to moisture penetration, albeit to a much lesser extent than commonly used polyimide films.
A xe2x80x9cmultilayer compositexe2x80x9d according to the present invention refers to composite structures having additional printed circuits and/or coverlayers applied to a circuit protective composite according to the present invention.
The term xe2x80x9cinfusionxe2x80x9d refers to polymer mobility between homogeneous polymeric films in a laminated condition during timed application of heat and pressure to produce a diffuse band of continuous polymeric phase that has no clearly defined demarcation boundary.
An xe2x80x9cinfusion layerxe2x80x9d also referred to herein as xe2x80x9ca continuous phasexe2x80x9d is the diffuse band resulting from infusion of laminated polymers under the influence of pressure at elevated temperature.
The beneficial effects described above apply generally to the exemplary devices and mechanisms disclosed herein of the circuit protective composites. The specific structures through which these benefits are delivered will be described in detail hereinbelow.